Disc-drive devices that store massive amounts of data are key components of computer systems. A disc drive typically includes a disc assembly having at least one disc that is rotated, an actuator that moves a transducer to various locations over the rotating disc, and circuitry that is used to write and/or read data to and from the disc via the transducer. The disc drive also includes circuitry for encoding data to be written to the disc so that it can be successfully retrieved from the disc and decoded. A microprocessor is typically used to control most operations within the disc drive, and to transmit read data to an external computer (or information processing system) and receive data from the external computer to write to the disc.
The disc drive includes a transducer for writing data onto circular or spiral tracks in a magnetic layer on the disc surfaces and for reading the data from the magnetic layer. In some drives, the transducer includes an electrically driven coil (or “write head”) that provides a magnetic field for writing data, and a magneto-resistive (MR) element (or “read head”) that detects changes in the magnetic field along the tracks for reading data. Each disc surface is typically allocated into a number of concentric circular tracks and data is stored along these tracks as individual magnetized patterns along the track. The transducer having a flux path and a gap is used to magnetize the track. The gap is passed near the disc. By changing the magnetic flux passing across the gap, individual portions of the track are magnetized. The same transducer is also used to read the data from the disc. Some transducers include a electromagnetic coil write head and a magneto-resistive read head.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (“ABS”) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Encoded information representative of data is stored on the tracks on the surface of the disc. A transducer, (read/write heads attached to a slider), is typically located on each side of each storage disc, and reads or writes information on the disc surface when the transducer is accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is moved to and held over a target track based on servo information read from the disc surface. As the storage disc spins and the read/write head is accurately positioned above a target track, the write head serially stores data onto a track by magnetically writing encoded information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track.
During manufacture, servo information is encoded on the disc to define tracks, and is subsequently used to accurately locate the transducer on these tracks to write and read data. Some drives use a dedicated servo surface to define tracks for all the other disc surfaces, while modem disc drives typically use embedded servo signals, where every disc surface includes its own servo information interspersed with data information. The written servo information is used during normal write and read operations to locate the actuator and its arm assembly and transducer head(s) at the required position on the disc surface and hold them very accurately in position during a read or write operation. The servo information is written or encoded onto the disc with a machine commonly referred to as a servo-track writer (STW) that is removed from the disc drive after the servo writing is completed. At the time the servo information is written, the disc drive is typically at the “head disk assembly” (HDA) stage. The HDA includes most of the mechanical drive components but does not typically include all the drive electronics. During the track writing process, the STW precisely locates the transducer heads relative to the disc surface and writes the servo information thereon. Accurate location of the transducer heads is necessary to ensure that the track definition remains concentric. If the servo track information is written eccentrically, the position of the transducer head during subsequent operation will require relatively large, constant radial adjustments in order to maintain placement over the track center. When the tracks are sufficiently eccentric, a significant portion of the disk surface must be allotted for track misregistration.
When the disc drive is manufactured, it is typical to write servo tracks on the disc surface in a predetermined pattern as described above. The encoded pattern of these servo tracks are later read and used to move the read/write heads to, and maintain the read/write heads on, a desired track of data. Typically, there will be a slight radial movement required to move either the read head or the write head to the centerline of the desired track. That is, once the position of the track is determined by reading the servo information, the read head (which was used to read the servo information) can be positioned to the centerline of the track, however a separate adjustment of the radial position may be required if the write head is to be positioned to the centerline of the track. The spacing between the read and write heads can vary from head to head because of manufacturing variations and differences in the angle which different heads are attached to their respective arms. Further, a rotary actuator moves the read and write heads in an arc, rather than along a radius of the disc, and thus the spacing between the read and write heads will vary depending on the radius of the track being accessed.
One design goal is to increase the storage capacity of disc drives. One way to increase capacity is to increase the density of tracks on the disc, and the density of date within each track, in order to save space and reduce the number of discs needed to store a particular amount of data. Several methods are currently used to store data on a disc. One method divides the disc surfaces into frequency zones, and groups one or more contiguous tracks into each frequency zone. Within each frequency zone, the transducer writes the data onto the disc at a fixed frequency as the magnetic disc rotates at a fixed angular velocity. The performance (in terms of the maximum frequency that data can be written and read reliably) of each frequency zone is determined, in order to determine the frequency that will be used for that frequency zone. Typically, a plurality of frequencies are tested, and the highest frequency giving reliable data transfer to all tracks within the frequency zone is used as the frequency for the entire frequency zone.
During manufacture of a disc drive, one or more sectors are typically discovered that have errors (initial defective sectors). One spare sector is typically allocated to replace each bad sectors discovered during manufacture. The initial defective sectors are marked as permanently unreadable, and are thereafter never used.
Disc drives are made more useful by defining the sectors on the drive using logical block addresses (LBAs). The logical block addresses are sequential sector numbers starting at zero or one and going up to a maximum LBA determined by the nominal capacity of the drive. An LBA provided to the drive is then translated to a selected disc position, for example, to the cylinder, head, and sector within the selected track that the sector will be found.
The process of reallocating spare sectors are to replace defective sectors complicates the process of translating LBAs to physical locations. There is, therefore, a need for a method and apparatus to allocate spare sectors and to translate addresses that improve the reliability, available storage space, and performance of a disc drive.